This application claims benefit of priority to Japanese Application No. JP2001-69633 filed Mar. 13, 2001, the entire content of which is incorporated by reference herein.
1. Field of the Invention
The present invention relates to a power converter for converting DC power into AC power or AC power into DC power, and more particularly, to a control device for a power converter which reduces loss generated by power semiconductor elements used in the power converter (loss through heat energy).
2. Description of the Related Art
FIG. 1A shows a composition of a conventional power converter. Members 1-6 are insulated gate-type bipolar transistors (hereinafter, xe2x80x9cIGBTxe2x80x9d), flywheel diodes (hereinafter, xe2x80x9cFWDxe2x80x9d) 7-12 being connected respectively inversely in parallel with the IGBT elements 1-6, to form a three-phase bridge circuit. 13 denotes a DC power supply, and 14 denotes the load of the power converter.
FIG. 1B shows a pulse width modulation (PWM) controller, which performs pulse width modulation of U-phase-W phase voltage references output by a control circuit (not illustrated) to convert same respectively into drive signals for the aforementioned IGBT 1-6.
The description here relates to IGBT, but it is also possible to use other power semiconductor elements, such as GTO (Gate Turn On Transistor), MOSFET (Metal Oxide Semiconductor Field Electric Transistor), bipolar transistors, or the like.
FIG. 2A illustrates the U-phase which is one phase of the three-phase bridge circuit, and FIG. 2B shows the composition of one phase of the PWM controller 15. The V-phase and W-phase also have a similar composition. In FIG. 2B, numeral 16 denotes a PWM carrier signal generator which generates a PWM carrier signal, and signal from the PWM carrier signal generator 16 is compared with the voltage reference signal by a PWM comparator 17.
The signal from the PWM comparator 17 is inverted by a signal inverter 18 to generate the signal for IGBT 2. Numerals 19 and 20 are respective dead-time generators for preventing a simultaneous xe2x80x98onxe2x80x99 state of the IGBT 1 and IGBT 2, due to the switching delay time of the IGBT 1 and IGBT 2.
FIG. 3 is a timing chart showing an example of the operations performed when a current iu flows in the direction of the arrow in FIG. 2A, against a time axis.
Hereinafter, the values iu, iv, iw, Vu, Vv, Vw, Eu, Ev, Ew, and the like, are used, and these are all vector values. For the sake of convenience, they are stated as scalar quantities.
In FIG. 3, the frequency of the voltage reference is zero, in other words, when the DC current is on, the voltage reference is positive, and if the current is flowing in the direction of iu, then current will only flow in IGBT 1 and FWD 8. Therefore, in this case, considering the U-phase alone, a loss due to the flow of current and switching is generated in IGBT 1 and FWD 8, whereas virtually no loss is generated in IGBT 2 and FWD 7, since no current flows therein.
For example, assuming that current iu=1, and taking the loss of the IGBT 1 per turn-on switching operation as Eon, the loss of the IGBT 1 per turn-off switching operation, as Eoff, the on voltage, as VCE, the time ratio that IGBT 1 is on (hereinafter, called xe2x80x9cdutyxe2x80x9d), as a (where 0xe2x89xa6axe2x89xa61), the loss of the FWD 8 per switching operation, as Edsw, the on voltage, as VF, and the frequency of the PWM carrier as fPWM, then the loss PT of the IGBT 1 and the loss PD of the FWD 8 are given by the following equations.
PT=(Eon+Eoff)xc3x97fPWM+VCExc3x971xc3x97a
PD=Edswxc3x97fPWM+VFxc3x971xc3x97(1xe2x88x92a)
The aforementioned losses, Eon, Eoff, Edsw, and the on voltages VCE, VF vary respectively with the current I.
If the load 14 is an electromotive device, then generally the resistance is low and the respective reference voltages are virtually zero, and consequently, the respective power semiconductor elements will be on for approximately xc2xd of the PWM cycle, and will be off for the remaining approximately xc2xd of the cycle (a≈0.5).
Furthermore, generally, the switching losses Eon, Eoff of the IGBT are large compared to the switching loss Edsw of the FWD, so the loss PT of the IGBT will be greater than the loss PD of the FWD. In other words, the loss of IGBT 1 will be greater than the loss of the FWD 8.
Consequently, the IGBT 1 generates a large amount of heat, and if the temperature rise caused by this heat generation is excessive, then there is a risk of damage to the IGBT 1, and hence problems arise in that when passing DC current, either the current of the IGBT 1 must be reduced, or the size of the cooling device (not illustrated) of the IGBT 1 must be increased in order to deal with the generated heat. If the current is reduced, then the current capacity of the device cannot be exploited satisfactorily.
Even if the voltage references are AC, at low frequencies, they can be regarded effectively as DC voltages, and hence similar problems to the foregoing arise.
Accordingly, one object of the present invention is to provide a novel control device for a power converter, whereby loss in power semiconductor elements can be reduced when passing DC current or low-frequency AC current.
In order to achieve the aforementioned object, the present invention is composed as follows. Namely, in a control device for a power converter constituted by power semiconductor elements for converting DC voltage to AC voltage, and AC voltage to DC voltage, by means of pulse width modulation, the loss of the aforementioned lower semiconductor elements constituting the power converter is reduced by adding a DC offset voltage to the AC voltage references, when the frequency of said AC voltage references is at of below a prescribed value.